Email is stored on the phone so can be viewed without wi-fi. As for space, I doubt they take up too much on the device. After-all, the majority of the time they are going to be predominantly text.

After a quicklook through my Finder on my iPod Touch. Mail appears to be stored here - var/root/Library/Mail/*Account Name*/Messages. I have about 60 messages on the device and a quick glance at the stats shows my account size at about 131kb.

Someone a bit geekier in storage and email who may want to look deeper in to see how they are stored etc could tell you more but the short answer to your thread is - Not Much

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